Thermal energy storage can be used to balance energy demand between day time and night time, or between different seasons such as summer and winter. Water is known to be used for storing solar heat (e.g. using a solar boiler) however this type of heat storage has a low energy density and thus requires a large water tank. Thermochemical materials (TCM) are a more compact way to store heat. The working principle is based on a chemical reaction between two compounds A and B with heat release (exothermic reaction), whereby another chemical compound C is formed. When this C compound is subjected to decomposition by heat during summer, two compounds A and B are formed. In the winter, these two compounds are allowed to react so that the stored heat is released and used for, e.g. building heating or domestic hot water. Apart from their compactness, further benefits include that no thermal insulation is required for the duration of the energy storage.
Especially salt hydrates are promising materials for heat storage. The reaction is then as follows:Salt.(m+n)H2O+heatSalt.mH2O+nH2Owherein m can be 0 or larger, and n>0. The term thermochemical compound in the context of the present application refers to any of “Salt.mH2O” “Salt.(m+n)H2O”, or “Salt”. It can thus be either an anhydrate or a hydrate.
A problem of the use of salt hydrates for thermochemical storage is however insufficient physical, mechanical and chemical stability of the material. Stability problems with TCM are, for example, corrosion of the environment, physical structural changes of the material (e.g. flaking off, coagulation, running, pulverization, fracture), chemical structural changes of the material (such as decomposition, leading to corrosion and toxic byproducts). The material should be hygroscopic on one hand, and have a good cyclability (be well regenerable) on the other hand. One of the problems with cyclability is for example melting of the material, or dissolution at uptake of water. Especially hygroscopic deliquescent salts have this problem. Melting and dissolution reduce the bed porosity of the material and thereby the vapour transport through the bed, which decreases the speed of water uptake during repeated uptake of water. Thereby the power input and output of the material are limited. Besides, for instance, pulverization of TCM grains leads to fragments of varying sizes, enabling close packing and lowering the empty volume fraction of the bed and reducing vapour transport.
Also, a problem of some TCM is that they are fully dehydrated (charged) only at very high temperatures, such as over 200° C., which means the use of full capacity of these materials is not possible in conventional applications such as in buildings, greenhouses, etc wherein much lower charging temperatures are employed.
It is therefore desired to provide a TCM that has an improved physical, mechanical and chemical stability. In addition, it is desired to provide a material that has an improved cyclability with respect to water uptake and release. It is also desired to provide TCM that have a low dehydration end temperature and can thus be used in full capacity at lower temperatures, e.g. under 150° C., for solar heat storage in buildings.